Force can cause an object to change its velocity or shape.

Forces push, pull, tug, heave, squeeze, stretch, twist or press.

Forces can change:

The shape of an object

The movement of an object

The speed of an object

The direction of an object

Not all forces can be seen but the effects can be measured. Forces can either

be contact forces, where the force needs to be in contact with the

object experiencing the force OR non-contact forces that will act

on an object from a distance without touching it.

Units of Force, Motion and Energy in Science

Quantity What is it measured in? Symbol Equipment used

Force (including weight) Newton N Spring balance

Mass kilograms kg scales

Velocity / speed metres per second ms-1 Ticker timer

Acceleration

(including gravitational acceleration )

metres per second per

second

ms-2 Ticker timer

Energy (including Work) Joules J

Contact and non-contact forces

Pushes, pulls, friction and tension are contact forces. Whatever causes the force actually touches the

object it acts upon. Non-contact forces such as electrostatic forces, magnetic forces and gravitational

forces act without contact between the object.

Forces and Motion Junior Science

Gravity is a force, which acts between bodies even though they are not in contact

Objects create a gravitational field around them. Gravity gives objects of mass in the field a weight

force.

the bigger the object; the stronger the field

the further away from the object, the less gravitational pull

Any other object within the field is pulled to the center of the mass:

>accelerating

>feeling weight

When gravitational force is acting on an object then we can say the object has weight force

Thrust force

Thrust (or applied force) requires some parts of an object (whether gas, liquid or solid) being pushed

forcefully from itself (rocket fuel from a rocket, for example). Once the rocket has left, the "thrust" is

no longer present. It also requires reaction (actual touching) of the thrust medium against the object.

Acceleration is the state of an object, due to a force applied. It is dependent on the force, and on the

mass of an object, but is not a force itself.

Friction force opposes an object that is experiencing thrust force. Thrust and friction are “paired

forces” that act in opposite directions on an object.

Friction often provides opposing force acting on moving bodies

Support forces

Sometimes friction is useful, at other times it is unhelpful.

Friction is a force that opposes motion. If an object has

no motion then there is no friction.

When friction occurs, and one surface moves against

another, the movement causes Kinetic energy to be

changed into heat energy.

Smooth surfaces create less friction than rough

surfaces. Friction that occurs between air or water and a

solid body is called resistance.

If friction and thrust forces are equal and opposite then

they are said to be balanced.

Support forces are equal and

opposite to an object experiencing

weight if the forces are balanced.

Support force in air is called lift and

in water is called buoyancy.

Buoyancy is an upward support

force caused by a fluid that opposes

the weight (gravitational force) of an

object in the fluid, usually water.

Once the object remains at a set

depth then the support force and

weight force are balanced.

Balanced forces

If pairs of forces acting on an object are equal and opposite, they are said to be balanced. The length

of the arrow shows relative magnitude of the force. The arrows must start from the middle of the

object.

Note: when an object is stationary there are only 2 forces acting upon the object; support and weight

force. There is no thrust or friction force

Unbalanced forces change motion

Balanced forces cause no change in speed or direction, since they exert equal, but opposite,

push/pull effects on an object. However, Unbalanced forces can change the speed and/or direction of

an object. Unbalanced forces occur when opposite forces are of a different magnitude (size)

Rules of Force Diagrams

Net Force

Calculating Net Force

The net force can be calculated by subtracting the smaller force from the larger force. If the forces are

pointing in the same direction, the forces add, giving a larger net force. If the forces are in opposite

direction, the forces subtract, giving a smaller net force (including a zero net force).

We use force diagrams to show the direction and magnitude (size)

of a force.

Force diagrams have rules:

The arrows showing a force must start (preferably) from the

centre of an object, but at least touching it.

Pairs of forces, such as support and weight, must be directly

opposite each other

Arrows must be pointing out.

The length of an arrow indicates magnitude of a force.

More force=longer arrow

Pairs of balanced forces have equal length arrows.

Pairs of unbalanced forces have different length arrows

A net force is the resultant force when

multiple forces interact. When forces are

balanced on an object, the net force is zero.

If there is zero net force, the object

maintains constant speed or is stationary.

An object experiencing unbalanced force will

have a net force greater or less than zero

and will accelerate in the direction of the

largest force.

If the net force is pointing in the same

direction as the direction of motion, the

object accelerates. If the net force is pointing

in the opposite direction to the direction of

motion, the object decelerates.

Net force = 120N – 30N = 90N accelerating the object from right to left (forward)

Force, mass and acceleration

Acceleration of a body depends both on its mass and on the size of the unbalanced force acting on it

Force = Mass x Acceleration

If the same amount of force is applied to two similar objects that have different mass, then then

smaller object will accelerate faster.

F = ma calculations

Ben is able to push both the car and the lawn mower so they accelerate at 0.5ms-2. The mass of the

car is 950kg and the mass of the lawn mower is 10kg. What is the force required to accelerate the car

compared to the lawn mower?

Car lawn mower

F=ma F=ma

F=950kg x 0.5ms-2 F=10kg x 0.5ms-2

The Force experienced by an object can be calculated by multiplying

the mass of the object by its acceleration.

Force = Mass x Acceleration

If more force is applied to an object then it will accelerate faster

F= 475N F= 5N

Mass and Weight

All objects have Mass. Mass refers to the amount of atoms, or substance, in an object. The formula

symbol for mass is m.

Mass is measured in kilograms (kg). 1kg = 1000g

The mass of the object remains the same regardless of its location.

Measuring Mass and weight

Weight is the downward force due to gravity

that an object experiences due to its mass. The

weight of an object depends on its location and

the gravity pulling down on it. The weight of an

object can change depending on where it is

located. Astronauts weigh less on the moon

because the force of gravity is less, but their

mass is the same in both locations. The formula

symbol for weight is Fw (weight force). Weight is

measured in Newtons (N)

Weight can be measured with a

spring balance, where the mass

can vertically hang and the

weight can be read off the force

meter. The scale will be in

Newtons (N).

A 2kg mass would read as (2 x

10ms-2) 20N

Mass can be measured with

scales, where the mass can sit

on top and the mass can be

read off the meter. The scale will

be in kilograms kg (or grams)

The Earth is the source of a gravitational field

The mass of the Earth creates an acceleration of 10 ms-2 for objects falling towards it. Regardless of

the size of the object, they all fall with the same acceleration - only the shape, which causes changes

in air resistance, causes some objects to experience more opposing force and accelerate slower.

To calculate our weight, which is a force on an object in a gravitational field, we multiply our mass by

the gravitational acceleration of Earth. On Earth, due to the size and mass of the planet, we

experience a gravitational pull of 10ms-2

This means if we were to freefall to Earth, every second we would accelerate 10m more per second – 1

second fall 10m, the next second fall 20m, the next second fall 30m etc.

Simple machines (Levers)

Levers are a simple machine that increase force

For a tool to be classed as a lever there must be:

a rigid handle

a fulcrum (or pivot) around which the handle rotates

a force increase – caused by the distance from the effort force to the fulcrum being larger

than the load force to the fulcrum

Simple machines can change the direction

or size of a force by using ‘mechanical

advantage’ to multiply force. A lever is

balanced on a fulcrum, which allows it to

pivot. A load is lifted by placing effort on

another part of the lever. A lever involves

moving a load around a pivot using effort

(or a force).

Examples of tools that are classified as

levers include scissors, pliers, hammer

claws and tongs.

Levers are a simple machine that increase force

Levers are classified in classes depending on the position of the effort and load in relation to the

fulcrum.

Seesaw type Lever (Class 1): A lever where the load force acts on the opposite side of the

fulcrum to the effort force. Examples include a Crowbar, Hammer and Tyre iron

Wheelbarrow type lever (class 2): A lever where the load force acts on the same side of the fulcrum as

the effort force. Examples include a Wheelbarrow, a Spanner and a Ratchet/tiedown

Inclined Planes

An inclined plane is a simple machine and it can be used to reduce the effort required to move a

load. If the slope has a small angle, then a person has to push or pull the object over a longer

distance to reach a height, but with very little effort. If the slope is steep, with a greater angle, a

person has to push or pull the object over a very short distance to reach the same height, but with

more effort. Mechanical advantage is calculated by length of slope divided by height of the slope.

There is a greater mechanical advantage if the slope is gentle because then less force will be needed

to move an object up (or down) the slope.

The different types of motion

Objects that move from one point of space to another over time are said to have motion. Examples

include a tortoise slowly moving across the ground or a bullet moving fast after it has been fired from

a gun. Objects that remain at the same point of space over a period of time are called stationary.

Examples include a person sitting still on a chair or a parked car.

Measuring Motion in Science

Quantity Unit Symbol Equipment used

Distance Kilometre km odometer

Metre m Metre ruler

millimetre mm Hand ruler

Time Hour hr clock

minute min watch

second s Stop watch

Speed

Speed is a measure of the distance travelled over the time taken. The more distance covered by an

object during a given time, the faster the speed it is moving. In this unit we use the term velocity to

mean the same thing.

Constant speed occurs when the object travels the same amount of distance at each even time

period. When we travel on an object moving at a constant speed, we do not feel movement for

example travelling in an airplane. Only when we observe other objects moving at a different speed to

us do we notice that we are moving.

Calculating speed

We use this formula to calculate speed by placing in the information we have about distance /time

into it. We can also rearrange the formula to calculate distance or time, as long as we know the other

two values. It is important to also use the units after any value in Science.

The relationships between distance, time and speed

Speed calculations

This formula will be given with all assessments (but not what the

letters stand for or the units)

and you will need to learn where to apply it.

Triangles can be used to calculate

speed, distance or time.

Cover the part of the triangle you wish

to calculate and multiply or divide the

remaining two values.

The unit for speed depends upon the

units for time and distance but the

most common unit in the lab is metres

per second (ms-1)

A football is kicked and during the first 2s it travels 36m. What

speed is it going during this time?

v=d/t

v=36m/2s

v=18ms-1

Average speed and instantaneous speed

Motion can be represented graphically - Distance vs Time

Distance (y-axis) and time (x-axis) data can be plotted on a graph to show patterns and compare

speeds. The steeper line on the left shows student A has a faster speed than student B.

A straight diagonal line indicates constant speed. A straight horizontal line indicates the object is

stationary.

Interpreting Distance/time graphs

A distance time graph can also show acceleration with a curved line (blue) because at each time

interval the distance travelled becomes larger and larger.

Changes in speed are also shown with a combination of diagonal and horizontal lines (red).

We calculate average speed (velocity). That is the speed

that has been travelled on average over the entire

distance. In a car the odometer measures instantaneous

speed. This is the speed that the car is travelling at in

that particular moment.

The average speed a car may have been travelling at for

a journey from Cambridge to Hamilton may have been

70km per hour but at some times they may have been

travelling at 100km per hour and at other times they may

have been travelling at 45km per hour.

We use the symbol ∆ to mean “change in”. So using the

formula we calculate the average velocity by dividing the

change in distance by the change in time taken.

Distance / time graph – Describing motion

Q1: The cyclists journey was plotted on the distance / time graph below. Describe the motion of the

cyclist in each of sections A,B,C and D

Section A: Increasing speed / accelerating

Section B: Constant speed

Section C: Decreasing speed, decelerating

Section D: Stopped / stationary

Gradients can be calculated from a Distance-time graph

The gradient of a distance - time graph can be used to calculate speed (velocity). The co-ordinates of

a straight line in the graph are taken (for example from A to B) by projecting back to the x and y axis.

Q2: Calculate the cyclists

speed during section B.

v = d / t

= 10 / 5

= 2 ms-1

Q3: what is the total

distance covered from 5 to

15seconds?

19m – 5m

= 14m in distance covered

Acceleration is a change in velocity

Objects that have a change in velocity are said to have acceleration. An increase in velocity or a

decrease in velocity (deceleration) are both types of acceleration.

A change in direction while travelling a constant speed is also acceleration. We notice when we are

travelling on an object that is accelerating by experiencing a change in gravity or G-force.

The units for Acceleration depend on what velocity and time are measured in. If time is measured in

seconds (s) and velocity is measured in metres per second (ms-1) then the units for acceleration will be

metres per second per second (ms-2)

To calculate the value for

time find the difference

between t1 and t2 by

subtracting the smallest

value from the largest value.

This will be your ∆ time.

Repeat to find distance on

the y axis. This will be your ∆

distance.

Place both values into your

formula to calculate speed

(velocity) v = ∆d/ ∆t

Acceleration or Deceleration

If an object is changing in speed and that change is positive, then the object is speeding up. When

calculating a value we can place a + sign in front of it if we wish.

If an object is changing in speed and that change is negative, then the object is slowing up.

When calculating acceleration we need to show this with a – (negative sign) in front of the value.

Alternatively if we clearly state the value is deceleration then we can leave the – sign off.

Acceleration can be calculated from a speed-time graph

Use the start and finish points of the time and the velocity to work out the total change. If the time

starts from 0 use that as your start point.

Acceleration Calculations

The BMW 135i is a formidable sports car, accelerating from 0kmhr-1 to 97kmhr-1 in 4.6 seconds. What

is the acceleration of this car in ms-2?

Motion can be represented graphically – Velocity vs Time

A velocity time graph can show acceleration with a diagonal line.

Constant velocity is shown with a straight horizontal line.

Values can be taken from the graphs and used to calculate acceleration.

The blue line shows a velocity of 10ms-1 travelled in 2 seconds.

The acceleration would therefore be:

a = ∆v/ t

= 10/2

a = 5ms-2

97kmhr-1 /3.6 = 26.9ms-1

aave=∆v/∆t

aave=26.9ms-1/4.6s

aave=∆v/∆t

aave=5.9ms-2